1. Field of the Invention
The present invention relates to L-aspartyl-L-phenylalanine (hereafter simply referred to as "AP") which is a precursor of L-aspartyl-L-phenylalanine methyl ester (hereinafter simply referred to as "APM"). APM is a dipeptide sweetener that has attracted attention in recent years.
2. Discussion of the Background
Known methods for the synthesis of a dipeptide utilizing a microorganism or an enzyme involve a process which comprises the introduction of appropriate protective groups to functional groups which are not involved in the condensation reaction of the two amino acids to restrict the location of the condensation reaction. A microorganism or an enzyme which is capable of synthesizing a peptide bond between these amino acid derivatives is then used to obtain the condensation products followed by removal of the superfluous protective groups from the condensation products to give the product dipeptide. Using this process, a satisfactory yield is obtained but the introduction and removal of the protecting groups is required. Thus, expensive protecting groups are needed and unnecessary complicated reaction steps are required.
For the biochemical syntheses of APM, there are known, for example, processes which comprise using a benzyloxy group (--Z) as a protective group for the amino group of aspartic acid and condensing N-benzyloxy-L-aspartic acid and phenylalanine methyl ester by the action of an endo-type protease. These processes involved the drawbacks described above.
Alternatively, processes for biochemical synthesis that require no protective groups in the synthesis of APM are also known (see Published Unexamined Japanese Patent Application 126796/83). Processes of this type do not require protective groups but the yield is poor and there is a problem in the synthesis of APM.
On the other hand, in the chemical synthesis of AP, it is necessary to use L-aspartic acid and L-phenylalanine, in order to produce sweet APM. This is because only L-aspartyl-L-phenylalanine methyl ester (APM), composed of only L-amino acids, is sweet among the four optical isomers of aspartyl-phenylalanine.
A good method for the separation of these four optical isomers was unknown to date so that DL-amino acids which were less expensive than L-amino acids could not be used as raw materials.
In addition, in order for the reaction to proceed, it is necessary to shift the reaction equilibrium toward the disadvantageous condensation side for the synthesis of AP. In the enzymatic condensation of L-aspartic acid and L-phenylalanine, it is necessary to remove AP from the reaction system to shift the synthesis equilibrium reaction toward the synthesis of AP. For this reason, processes utilizing these condensation reactions remove AP either by adsorbing the AP produced onto ion exchange resins, or by incorporating a precipitating agent in such a manner that AP becomes selectively insoluble.
These prior art processes have several serious defects, namely poor yields in conventional biochemical syntheses requiring no protecting groups and the use of expensive protecting group reagents and unnecessary reaction steps in processes involving protection of functional groups.